Law of Constant Proportion
As we know, compounds are composed of two or more elements. The proportion in which elements are present in a compound remains the same, irrespective of its method of preparation.
For example, pure water obtained from any source (well, river, lake, or sea) and from any country (India, Russia, America, etc.) will always contain two hydrogen atoms and one oxygen atom. Hydrogen and oxygen combine together in the ratio of 1:8 by mass respectively to form water, and the ratio by the number of atoms for water will always be H : O = 2 : 1. Thus, 18 g of water contains 2 g of hydrogen and 16 g of oxygen.Similarly, in ammonia, nitrogen makes up 14/17 of the mass of ammonia, while hydrogen makes up the remaining 3/17 of the mass. Irrespective of the source from which ammonia is obtained, it will always contain nitrogen and hydrogen in the ratio of 14: 3 by mass. Thus, 17 g of ammonia contains 14 g of nitrogen and 3 g of hydrogen, and 34 g of ammonia contains 28 g of nitrogen and 6 g of hydrogen. This led to the law of constant proportion.
Thus, according to the law of constant proportion, a chemical substance always contains the same elements in a fixed proportion by mass, irrespective of its source.
The law of constant proportion is also known as the law of definite proportion. This law was introduced by Proust, which stated that â€˜in a compound, the elements are always present in definite proportions by massâ€™.